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IC 


9002 



Bureau of Mines Information Circular/1985 



c 



^^V 



Production of High-Purity Gold From Zinc 
Precipitates and Steel Wool Cathodes 
by Hydrometallurgical Refining 

By G. E. McClelland, M. D. Wroblewski, and J. A. Elsele 




UNITED STATES DEPARTMENT OF THE INTERIOR 



75^ 



AMINES 75TH AV5J^ 



I r^-for f^iOL-K c^ c \ i^culcLT ( U A » red Stoies . S^nreau o-f Mi'^tes ) 
Information Circular 9002 

Production of High-Purity Gold From Zinc 
Precipitates and Steel Wool Cathodes 
by Hydrometallurgical Refining 

By G. E. McClelland, M. D. Wrobiewski, and J. A. Eisele 




UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 



/r\^ 



(\^ 



]}}< .kO^ 



fj 



& 



Op- 




Library of Congress Cataloging in Publication Data: 



McClelland, G. E 

Production of high-purity gold from zinc precipitates and steel wool 
cathodes by hydrometallurgical refining. 

(Information circular / United States Department of the Interior, Bu- 
reau of Mines ; 9002) 

Bibliography: p. 11. 

Supt. of Docs, no.: I 28.27:9002. 

1. Gold— Metallurgy. 2. Hydrometallurgy. 3. Precipitation (Chem- 
istry). 4. Zinc. 5. Cathodes. 6. Steel. I. Wroblewski, M. D. (Matt D.). 
II. Eisele, J, A. (Judith A.). III. Title. IV. Series: Information cir- 
cular (United States. Bureau of Mines) ; 9002. 

TN295.U4 [TN769] 622s [6(59'. 22] 84-600263 



s 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Experimental procedure. 2 

Acid pretreatment 3 

Precipitation of sliver 3 

Aqua regla dissolution of gold 3 

Precipitation of metallic gold 3 

Refining of precipitated gold. 4 

Cyanldatlon of acld-leached residues for recovery of residual precious metals 4 

Results and discussion 4 

Acid pretreatment 4 

Precipitation of silver 4 

Aqua regla dissolution of gold 5 

Precipitation of metallic gold 5 

Refining of precipitated gold 7 

Cyanldatlon of acid leached residues for recovery of residual precious metals 7 

Slurry solids for acid leaching procedures 8 

Recommended procedure for zinc precipitates 8 

Recommended procedure for steel wool cathodes 10 

Handling and disposal of acid solutions and acid wastes 10 

Summary and conclusions 11 

References 11 

ILLUSTRATIONS 

1. Flowsheet for chemical refining zinc precipitates. 8 

2. Flowsheet for chemical refining steel wool cathodes 9 

TABLES 

1. HNO3 pretreatment of zinc precipitates.. 5 

2. Precipitation of silver from HNO3 leaching solutions 5 

3. Results of aqua regla leaching of HNO3 residues,. 6 

4. Precipitation of gold by reduction with (C00H)2 6 

5. Precipitation of gold by reduction with SO2 7 

6. Results of flre-ref Inlng precipitated gold 7 

7. Results of cyanide leaching of aqua regia-leached residue 8 





UNIT OF 


MEASURE 


ABBREVIATIONS 


USED IN 


THIS REPORT 


°c 


degree ce 


Lsius 




mg/L 


milligram per liter 


g 


gram 






min 


minute 


h 


hour 






ml, 


milliliter 


kg 


kilogram 






oz/ton 


ounce per ton 


L 


liter 






pet 


weight percent 


M 


molar 











PRODUCTION OF HIGH-PURITY GOLD FROM ZINC PRECIPITATES AND STEEL WOOL 
CATHODES BY HYDROMETALLURGICAL REFINING 

By G. E. McClelland/ M. D. Wroblewski,^ and J. A. Eisele^ 



ABSTRACT 

The Bureau of Mines Investigated chemical methods for producing high- 
purity gold from precious-metal-bearing zinc precipitates and steel wool 
cathodes. Precious-metal-bearing zinc precipitates and steel wool cath- 
odes are unrefined products from conventional cyanidation and heap 
leaching-cyanidation operations. The zinc precipitates contained 14.40 
pet Au and 0.35 pet Ag. The precious-metal-bearing steel wool cathodes 
contained 20.65 pet Au and 4.84 pet Ag. The precipitates and cathodes 
were treated with dilute acid to solubilize the silver and/or base met- 
als. The gold-bearing residue was leached in dilute aqua reqia to solu- 
bilize the gold. High-purity gold was precipitated from the aqua regia 
solution with oxalic acid, sulfurous acid, sodium bisulfite, and gaseous 
sulfur dioxide. The leaching-precipitation experiments recovered 99.9 
pet of the gold. The gold precipitates ranged in fineness from 997 to 
999 fine. 

The chemical refining method provides a viable technique for the 
smaller operator to produce high-purity gold without using pyrometallur- 
gical refining methods. 

^Metallurgist, Reno Research Center, Bureau of Mines, Reno, NV (now with Hienen- 
Lindstrom Associates, Sparks, NV) . 

^Physical science technician, Reno Research Center, Bureau of Mines, Reno, NV. 
■^Supervisory chemical engineer, Reno Research Center, Bureau of Mines, Reno, NV. 



INTRODUCTION 



The increase in precious metals prices 
since 1975 has stimulated extensive ex- 
ploration programs. Concurrently, the 
Bureau of Mines (_3, 8^)^ has devised im- 
proved leaching technology for processing 
gold-silver ores. Many of the precious 
metal resources are too small to warrant 
expenditure of large amounts of capital 
to recover the contained values. The 
operators of small properties are limited 
in processing technology to low-cost 
heap leaching or small-scale agitation 
cyanidation. 

Regardless of the leaching technology 
employed, the dissolved precious metals 
are recovered from the pregnant solution 
either by precipitation on zinc dust 
(J^, _5, 9-10) or by carbon adsorption- 
desorption-electrowinning 0-2, A~5» 11~ 
12). In larger scale operations, the 
precious-metal-bearing zinc precipitates 
and steel wool cathodes are fire-refined 
to obtain dore bullion. The dore bullion 
is either sold to a refiner or refined on 



site to obtain pure gold and silver 
bullion. 

The operators of small mining proper- 
ties in many instances cannot afford to 
construct a precious metal refinery and 
must sell their zinc precipitates or 
steel wool cathodes to a custom refiner. 
Marketing their product in this manner 
decreases the small operators' profits 
because of loss of interest on the price 
of the metal while it is idle, assay 
charges, costs for packing, shipping, and 
insurance while in transit, and the re- 
finers' fees and profits. 

The objective of this investigation was 
to develop a hydrometallurgical refining 
procedure to enable the smaller operator 
to produce high-purity precious metal 
products from zinc precipitates or steel 
wool cathodes. The chemistry used in 
this procedure is essentially the same as 
used for chemically refining jewelry or 
dental scrap (6-7). 



EXPERIMENTAL PROCEDURE 



Gold- and silver-bearing zinc precipi- 
tate used in the experiments was obtained 
from a large operating gold mine. Anal- 
ysis of the zinc precipitate was as 
follows: 

Element pet 

Ag 0.35 

Au 14.40 

CaO 10.2 

CO2 and others 16.45 

Cu 9 

Hg 6 

Pb 2.1 

S 4.6 

Si02 25.7 

Zn 9.4 

Other metal oxides.. 15.3 

Analyses for specific elements and com- 
pounds contained in the zinc precipitate 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



were made by fire assay, atomic absorp- 
tion, and wet-chemical methods. Spectro- 
graphic analysis showed that the zinc 
precipitate contained small amounts of 
other metals. 

The wet zinc precipitate was air-dried 
at room temperature for 120 h. The dry 
precipitate was sized on a 100-mesh 
screen. The oversize was ground to pass 
100 mesh and mixed with the minus 100- 
mesh material in a glass bottle by roll- 
ing and tumbling. 

A precious metal-bearing steel wool 
cathode was generated from the Bureau's 
carbon stripping-electrowinning pilot 
demonstration unit (PDU) in Reno, NV. 
The cathode was oven-dried and weighed. 
Pieces were cut from different regions of 
the cathode and hand-blended to make a 
head sample. The sample was assayed by 
conventional fire assay methods. The 
steel wool cathode assayed 20.65 pet Au, 
4.84 pet Ag, and 0.14 pet Cu. 



Solid products (residues, Au sponge, 
AgCl precipitate, etc.) from all experi- 
ments were analyzed by fire assay for 
precious metal values. Solutions were 
analyzed by atomic absorption spectropho- 
tometry for metal values. 

ACID PRETREATMENT 

Acid pretreatment experiments were con- 
ducted on 25-g charges of zinc precipi- 
tate which were agitated for 2 h at 80° C 
in 200 mL of 6M acid. Acids used were 
H2SO4 , HCl, and HNO3 . After leaching, 
the pulps were filtered, and the residues 
were washed with 100 mL of deionized wa- 
ter. Residues were dried, weighed, and 
analyzed for residual metal values. 

A multistage HNO3 pretreatment experi- 
ment was conducted on a 25-g charge of 
zinc precipitate to improve silver recov- 
ery. The zinc precipitate was agitated 
for 2 h at 85° C in 100 mL of 6M HNO3 . 
After pretreatment, the pulp was filtered 
and the filtrate was analyzed for silver 
and base metal content. The residue was 
leached three times. The final residue 
was washed with 400 mL H2O and was as- 
sayed for silver, 

A 62-g steel wool cathode was digested 
at 90° C for 1 h in 1 L of 5M HCl to dis- 
solve excess iron. The solids were sepa- 
rated from the acid solution by filtra- 
tion. The solids were washed with dis- 
tilled water, dried, and saved for aqua 
regia leaching to recover the contained 
gold values. 

PRECIPITATION OF SILVER 

Silver precipitation experiments were 
conducted by adding known quantities of 
NaCl to slowly agitated silver-bearing 
HNO3 pregnant solution at room tempera- 
ture. When initial AgCl precipitation 
ceased, 1 g of additional NaCl was added 
and the solution was gently agitated. If 
further AgCl precipitation was observed, 
additional NaCl was added in 1-g incre- 
ments until precipitation was complete. 
The AgCl precipitate was separated from 
the acid solution by filtration, washed 
with distilled water, and dried. 



AQUA REGIA DISSOLUTION OF GOLD 

The gold-bearing residues from acid- 
preleaching zinc precipitates and steel 
wool cathodes (10 to 15.7 g) were agi- 
tated at 90° C for 1 h in dilute aqua 
regia. ^ After leaching, the slurries 
were filtered on glass fiber filter paper 
and washed with distilled water. 

A simulated two-stage aqua regia leach- 
ing experiment was conducted on an HNO3- 
leached residue. Ten grams of residue 
were agitated at 90° C for 1 h in 175 mL 
of dilute aqua regia. The pulp was fil- 
tered, and the filtrate was used to leach 
a fresh 10-g charge of residue at 90° C 
for 1 h. The slurry was filtered, and 
the washed residue was leached with di- 
lute aqua regia. 

PRECIPITATION OF METALLIC GOLD 

One-hundred milliliters of gold-bearing 
dilute aqua regia solution was heated 
to 80° C. The stoichiometric amount of 
oxalic acid (0,69 g (C00H)2 per gram Au) 
required to precipitate the contained 
gold was added slowly and gently agi- 
tated. The solution was digested at 
temperature until gold precipitation 
stopped. Additional (C00H)2 was added 
to the solution in 1-g increments until 
precipitation stopped and the bright yel- 
low color of the solution disappeared. 
The acid solution was decanted and al- 
lowed to stand overnight at room tempera- 
ture for additional gold precipitate to 
form. The gold products were combined, 
washed with distilled water, dried, and 
assayed. 

A second method for gold precipitation 
was to slowly bubble SO2 gas through a 
fritted glass dispenser into 100 ml of 
gold-bearing aqua regia solution at room 
temperature for periods ranging from 
15 min to 1 h. The gold precipitate was 
washed with distilled water, dried, and 
assayed. 

^Dilute aqua regia refers to a solution 
containing 75 mL of 36.5-pct HCl and 50 
mL of 70.0-pct HNO3 per liter of water. 



The third method evaluated was sodium 
bisulfite or sulfurous acid precipitation 
of gold from 100 mL of gold-bearing aqua 
regia solution. The sodium bisulfite 
(1.8 g NaHS03 per gram Au) or sulfurous 
acid (2.5 g H2SO3 per gram Au) was added 
to the solution in one increment. The 
mixture was gently stirred for 15 min at 
80° C. Precipitation was complete when 
no additional precipitation was observed 
and when the yellow color of the solution 
disappeared. The barren solution was an- 
alyzed for residual gold. The precip- 
itate was filtered, washed, dried, and 
assayed, 

REFINING OF PRECIPITATED GOLD 

Separate charges of gold precipitated 
by (C00H)2 or by SO2 gas were mixed with 
nitre (KNO3), silica (Si02), and borax 
(Na2B4 07) and heated in clay crucibles to 
1,000° C for 1 h. The molten charges 



were poured into cast iron molds and 
cooled. The gold beads were fire-assayed 
for fineness, and the slags were fire- 
assayed for residual gold content. 

In a separate experiment, gold precipi- 
tated with SO2 was upgraded in purity by 
digesting in 5M HNO3 at 90° C for 4 h. 
The gold sponge was washed with distilled 
water, dried, and assayed. 

CYANIDATION OF ACID-LEACHED RESIDUES 
FOR RECOVERY OF RESIDUAL 
PRECIOUS METALS 

Gold- and silver-bearing siliceous 
residue from aqua regia leaching of zinc 
precipitates (~7.5 g) was agitated at 
room temperature for 24 h in 100 mL H2O 
containing 0.3 to 1.0 g NaCN and 0.2 g 
CaO. The cyanide leached residues were 
filtered, washed, and dried. 



RESULTS AND DISCUSSION 



The chemical technique for refining 
zinc precipitates differs from the pro- 
cess for steel wool cathodes. For refin- 
ing zinc precipitates, the silver and 
base metals must be separated from the 
gold and silica. Since the steel wool 
cathodes contained no silica, separation 
of the gold from the silver by acid pre- 
leaching was not necessary. The residue 
generated from the aqua regia leaching of 
the HCl pretreated steel wool cathodes 
was a nearly pure AgCl product which 
assayed 74 pet Ag. 

ACID PRETREATMENT 

Preliminary tests showed that HNO3 was 
the only effective acid for dissolving 
the silver from the zinc precipitates. 
The results of HNO3 leaching experiments 
are shown in table 1. The data show that 
leaching with 6M HNO3 for 6 h dissolves 
76 pet of the silver and most of the base 
metals and that multiple leaches are not 
justified. 

HCl pretreatment of the steel wool 
cathodes generated an acid filtrate which 



contained 99 pet of the Fe, 28 pet of the 
Cu, 0.016 pet of the Au, and 1.7 pet of 
the Ag. The leached residue was 80.5 pet 
Au, 19.0 pet Ag, 0.4 pet Cu, and <0.1 pet 
Fe. 

PRECIPITATION OF SILVER 

Silver can be precipitated as AgCl 
chloride from the HNO3 leaching solutions 
with NaCl according to the following equ- 
ation: 

NaCl + AgN03 ^ AgCl(s) + NaN03 . (1) 

The amount of silver contained in the 
HNO3 leaching solutions must be known to 
ensure the addition of the proper amount 
of NaCl. The results of NaCl precipita- 
tion of silver from HNO3 leaching solu- 
tions with stoichiometric and excess NaCl 
are shown in table 2. The silver was 
precipitated, but a large excess of NaCl 
was needed. 

The dry AgCl precipitate is suitable 
for fire refining, or is marketable if it 
contains between 70 and 75 pet Ag. 



TABLE 1. - HNO3 pretreatment of zinc precipitates 



Acid leaching 


HNO3, 
M 


Extraction, pet 


number 


Ag 


Cu 


Fe 


Pb 


Zn 



SINGLE LEACHING WITH 200 mL OF SOLUTION 





1.5 
3.0 
6.0 
6.0 
6.0 


50.2 
45.5 
64.8 
69.1 
69.8 


97.8 
>99 
>99 
>99 
>99 


92.0 
>99 
>99 
>99 
>99 


72.4 
81.9 
85.7 
86.3 
85.9 


97.9 




>99 




>99 




>99 




>99 



MULTIPLE LEACHING WITH 100 mL OF SOLUTION 



1 


6.0 
6.0 
6.0 


17.0 

61.1 

.1 


86.9 

12.1 

.9 


48.5 

45.6 

5.8 


48.2 
27.9 
20.0 


89.4 


2 


10.2 


3 


.3 


Total 


NAp 


78.2 


>99 


>99 


96.1 


>99 



SINGLE LEACHING WITH 200 mL OF SOLUTION, 6 h 


1 6.0 75.9 >99 1 >99 96.0 >99 



NAp Not applicable. 



NOTE. — Leaching 
taled 6 h. 



time = 2 h, multiple leaching to- 



TABLE 2. - Precipitation of silver from HNO3 
leaching solutions 



Pet of 


Silver, mg/L 


Silver 


stoichiometric 
NaCl 


In pregnant 
solution 


In barren 
solution 


precipitated, 
pet 


100 


120 
120 
180 
120 
120 


110 

95 

56 

20 

2.1 


8.3 


500 


20.8 


1,000 


68.9 


1,500 


83.3 


3,000 


98.3 



AQUA REGIA DISSOLUTION OF GOLD 

Dilute aqua regia leaching experiments 
were conducted to determine if gold could 
be extracted from residues from the HNO3 
pretreatment of zinc precipitates and HCl 
pretreatment of steel wool cathodes. 
The amounts of HCl and HNO3 used were 
150 to 300 pet in excess of the stoichio- 
metric amounts required by the following 
equation: 

Au + 3HNO3 + 4HC1 

->■ HAUCI4 + 3NO2 + 3H2O. (2) 

The experimental results are shown in ta- 
ble 3 and indicate that agitated leaching 
in hot dilute aqua regia dissolved metal- 
lic gold from the acid-leached residues. 



A two-stage leach produced a pregnant 
AUCI3 solution low in residual HNO3, 
which improves the precipitation of gold 
from the aqua regia leaching solution. 

PRECIPITATION OF METALLIC GOLD 

(C00H)2 was added to the pregnant di- 
lute aqua regia solution to precipitate 
gold as metallic crystals, according to 
equation 3: 

3(COOH)2 + 2HAUCI4 

->■ 2Au + 6CO2 + 8HC1. (3) 

The experimental results for the chemi- 
cal reduction of gold with (C00H)2 are 
shown in table 4. 



TABLE 3. - Results of aqua regia leaching of HN03-leached residues 



36-pct HCl/70-pct HNO3, mL^ 



Pretreated 

residue 
weight, g 



Volume of 

dilute 

aqua regia 

used, mL 



Au 

recovered , 

g 



Au 
extracted, 
pet 



Residual 2 
HNO3, M 



Single leaching: 

10/7 , 

15/10 , 

2-stage leaching: 

Stage 1, 15/10 , 

Stage 2, NAp , 

Leaching of 2d-stage resi- 
due, 15/10 , 

Residue from HCl leach of 
steel wool cathodes, 50/30. 



5l2 
^12 

310 

Mo 

NAp 



15.7 



175 
175 

175 
175 

175 



500 



3.51 
3.74 

3.19 
6.63 

.14 



12.77 



91.6 
99.9 

99.9 
97.9 

100 
(total) 

99.75 



>0.5 
>.5 

.48 
<.l 

.88 



>.5 



NAp Not applicable. 

^ Pairs of figures in column entries are the respective amounts of 36-pct 
70-pct HNO3; e.g., 10/7 = 10 mL HCl and 7 mL HNO3. 
^Residual HNO3 determined by chemical analysis. 
■^Residue from HNO3 leaching of zinc precipitate. 

TABLE 4. - Precipitation of gold by reduction with (C00H)2 



HCl and 



Test 1 



Test 2 



Test 3^ 



Test 42 Test 5^ Test 6^ 



(C00H)2 added g.. 

Gold precipitated g. . 

Gold precipitated pet. . 

Fineness 

Gold in barren mg/L. . 

Urea added g.. 

NaOH added g . . 

Retention time h. . 

Residual HNO3 .M. . 

Final pH 



15 

1.48 

96.54 

999 

200 





24 

0.5 

<0 



20 

5.20 

98.95 

995 

300 





24 

0.5 

<0 



5 

1.05 

99.91 

999 

5 

2 



24 

<0.1 

<0 



15 

3.22 

99.43 

997 

110 





24 

<0.1 

<0 



10 

3.14 
99.99 
998 

3 


14 

2 
<0.1 

1.7 



11.6 

5.50 

99.95 

998 

4 

8 

3 

2 

<0.1 

1.0 



'Pregnant solution generated 
precipitates. 

2 Pregnant solution generated by 
zinc precipitates. 



by dilute aqua regia leaching of pretreated zinc 



2-stage dilute aqua regia leaching of pretreated 
Pregnant solution generated by dilute aqua regia leaching of pretreated steel wool 



cathode. 



NOTE. — All tests made at 70° to 80° C. 

The data show that reduction with 
(C00H)2 precipitated high-purity gold 
(998 to 999 fine) from dilute aqua regia 
leaching solutions. Five-tenths molar 
residual HNO3 in the leach solution re- 
sulted in slightly less precipitation of 
gold. Gold precipitation was higher from 
aqua regia leaching solutions from the 
second stage of the two-stage leaching 
experiment, which contained less than 



O.IM HNO3. Residual HNO3 
decreased by adding urea 
equation 4, 



could also be 
according to 



6HNO3 + 5CO(NH2)2 
->■ 8N2 + 5C02 + I3H2O, 



(4) 



increasing the percent of gold precip- 
itated. Increasing the pH with NaOH 



TABLE 5. - Precipitation of gold by reduction with SO2 



Test 1 



Test 2 



Test 3 



Test 4 



Test 5 



SO2 source 

Quantity used g . . 

Gold precipitated g . . 

Gold precipitated pet,. 

Fineness 

Gold in barren mg/L. . 

Reaction time min. . 

Residual HNO3 M» • 

^Weight of pure substance, 



NOTE. — Pregnant solutions were generated by the aqua regia leach- 
ing of Zn precipitates. Tests using SO2 gas were carried out at 
room temperature (25° C). Tests using NaHS03 and H2SO3 were heated 
to 80° C. 



Gas 

Excess 

2.39 

99.63 

988 

59 

15 

0.5 



Gas 

Excess 

1.55 

99.62 

983 

42 

15 

<0.1 



Gas 

Excess 

2.23 

99.61 

985 

67 

60 

<0.1 



NaHS03 

^2.9 

1.61 

99.44 

979 

71 

15 

0.5 



H2SO3 

I3 

1.15 

99.72 

970 

33 

15 

0.5 



decreased the time required to precipi- 
tate the gold. The crystalline gold pro- 
duced by (GOGH) 2 reduction is bright and 
lustrous. 

SO2 , NaHS03 , or H2SO3 was added to pre- 
cipitate gold as metallic sponge, accord- 
ing to equation 5, 

2AUCI3 + 3S02 + 6H2O 

-»- 2Au + 6HC1 + 3H2SO4. (5) 

The experimental results for the reduc- 
tion of gold with S02-producing compounds 

are shown in table 5. The gold precipi- 
tate was not as pure as gold precipitated 
with (C00H)2 and did not have the lus- 
trous appearance. 

REFINING OF PRECIPITATED GOLD 

If the precipitated gold is not of high 
enough purity, refining by smelting or 
leaching with hot HNO3 may be necessary. 
Gold precipitated by SO2 was upgraded by 
digestion in hot HNO3 from 985 to 998 
fine. The experimental results obtained 
from melting (C00H)2- or S02-precipitated 
gold with nitre, borax, and silica are 
shown in table 6. Gold produced by 
chemical refining can be purified, if 



necessary, to 999.9 fine by fluxing with 
nitre, borax, and silica at 1,000° C, 

TABLE 6. - Results of fire-refining 
precipitated gold' 





Test 1 


Test 2 


Test 3 


Weight, g: 

Nitre (KNO3).... 
Borax (Na2B4 07). 
Silica (Si02)... 
Au bead. ........ 


2.9 

3.8 

2.9 

0.99 

979 
997 


3.4 

3.2 

2.4 

0.99 

990 
999.9 


3.4 

3.2 

2.4 

0.99 


Fineness: 

Before. ......... 


995 


After 


999.9 



' 1-g gold samples, 

CYANIDATION OF ACID LEACHED RESIDUES 
FOR RECOVERY OF RESIDUAL 
PRECIOUS METALS 

The siliceous residues from leaching 
zinc precipitates contained residual pre- 
cious metals. The results of cyanidation 
to recover the precious metal values are 
shown in table 7, Approximately 96 pet 
of the gold and silver in the residues 
was recovered by leaching in alkaline 
cyanide solutions. The precious-metal- 
bearing residues could be recycled to the 
plant's cyanide leaching circuit. 



TABLE 7. - Results of cyanide leaching of 
aqua regia-leached residue 



NaCN g. 

Gold, oz/ton: 

Feed 

Tailing 

Gold extracted, ., ,pct. 
Silver, oz/ton: 

Feed 

Tailing 

Silver extracted. .pet. 
Time h. 



Test 
1 



1.0 

2.90 

0.13 

95.52 

171.2 

1.9 

98.89 

24 



Test 
2 



1.0 

14.00 

0.51 

96.36 

118.1 

1.7 

98.56 

24 



Test 
3 



0.2 

33.00 

1.00 

96.97 

196.8 

8.6 

95.63 

1 



SLURRY SOLIDS FOR ACID 
LEACHING PROCEDURES 

The slurry solids recommended for all 
the acid-leaching procedures are very low 
and depend on the amount of gold and/or 
silver contained in the specific feed. 
If the pulp solids are too high, the 
solubility limits for gold, silver, 
and other metals in the specific acid- 
leaching liquor may be attained, and pre- 
mature precipitation of precious metals 
and other metals may occur. The recom- 
mended pulp solids should be used unless 
it is determined by metallurgical testing 
that the slurry solids can be changed. 



RECOMMENDED PROCEDURE FOR ZINC PRECIPITATES 



The recommended flowsheets based on the 
results of this study are shown in fig- 
ures 1 and 2. Acid pre treatment of zinc 



precipitates is required to dissolve the 
silver and base metals and leaves a gold- 
bearing siliceous residue. One kilogram 



Zinc precipitates 




H2O 



Residue -^ 
recycle to 
cyanide 
leaching 



Urea- 
Oxalic acid- 
NaOH- 



HNO: 



H2O 



HgO 



Residue 



Aqua regia 

leaching , 

90° C. I h 



Filtration 



Filtrate 



Precipitation of 

nnetallic gold, 

80° C 



Acid 
pretreatment, 

85° C, 6 h 



n 



Filtration 



NaCI 



Filtrate 



H2O 

AgCI 
product 

H2O 
Gold sponge 



Barren 
acid to 
waste 



1 



Silver 
precipitation 



U 



Filtration 



Filtrate 
to waste 




Gold 
product 



FIGURE 1. - Flowsheet for chemical refining zinc precipitates. 



steel wool cathode 



HCI 
HgO 



H2O - 
Residue 




I 



Aqua regia 

leaching, 

90' C, I h 



H2O 



O 



Filtration 



Filtrate 



Urea 
Oxalic acid 

NaOH 




Precipitation of 

metallic gold, 

80* C 



Acid 
pretreatment, 
85" C, 6 h 



n 



Filtration 



-Filtrate 
to waste 



^ AgCI residue product 

H2O 
Gold sponge 




•Barren 
acid to 
waste 

FIGURE 2. - Flowsheet for chemical refining steel wool cathodes. 



Gold 
product 



of zinc precipitate is slowly added to 
10 L of 6M HNO3. The slurry is heated to 
85° C, agitated, digested at temperature 
for 6 h, and filtered hot. The residue 
is washed with water. The wash water and 
acid filtrate are combined and saved for 
silver recovery. 

The silver contained in the acid fil- 
trate and wash water is precipitated as 
AgCl by the addition of NaCl. A tenfold 
stoichiometric excess of NaCl is added to 
the HNO3 leaching solution at room tem- 
perature. (The stoichiometric amount of 
NaCl is 0.54 g NaCl per gram of silver in 
solution.) The solution is agitated 
gently for 5 min and filtered. One gram 
of NaCl is added to the filtrate. If 
additional AgCl precipitation is ob- 
served, the above procedure is repeated, 
NaCl is added in the above manner until 
no precipitation is observed. In some 



cases a 30-fold excess of NaCl may be re- 
quired. The AgCl product is washed with 
distilled water and dried at low tempera- 
ture (<100° C). 

Gold is recovered from the HNO3- 
preleached siliceous residue by leaching 
with aqua regia. The HN03-leached resi- 
due is divided into two equal charges. 
One charge is slurried with dilute aqua 
regia. For every 100 g of charge, 2 L of 
dilute aqua regia containing 150 mL of 
36.5-pct HCI and 100 mL of 70-pct HNO3 
are used. The solution is heated to 
90° C and digested with continuous agita- 
tion for 1 h. The slurry is filtered hot 
on glass fiber filter paper. The acid 
filtrate is removed, and the residue is 
washed with distilled water until no yel- 
low color can be seen in the filtrate. 
The washes are combined with the acid 
filtrate. The first aqua regia-leached 



10 



residue is saved for recycle to cyanide 
leaching. The acid filtrate is slurried 
with the second charge of HN03-leached 
residue and treated as above. The second 
aqua regia leached residue is saved for 
further dilute aqua regia leaching. 

Metallic gold is precipitated from the 
aqua regia leaching solution by (C00H)2. 
The pregnant AUCI3 solution (dilute aqua 
regia leaching solution) is heated to 
80° C and treated as follows: 

1. Slowly add urea prills in 1-g 
increments until gas evolution stops. 
Add the stoichiometric amount of (C00H)2 
required to precipitate the known or 
estimated amount of gold contained in 
the pregnant solution (0.69 g (C00H)2 per 
gram of gold in solution) . Precipita- 
tion should start within 10 min. If 
precipitation does not start, or if it 



stops, add enough 30-pct NaOH solution to 
bring the pH to 1,0, Digest the solution 
at 80° C until precipitation stops. 

2. Add 10 pet of the previously calcu- 
lated amount of (C00H)2. If precipita- 
tion is not observed, or if it stops, add 
enough NaOH to bring the pH up to 1.0, 
Digest the solution until precipitation 
stops. Repeat until the yellow color of 
the solution disappears. Combine the 
gold precipitate into a single sponge by 
using a spatula to free entrained gases 
and to scrape the sides and bottom of the 
vessel. When all of the gold has been 
collected, decant the barren solution and 
analyze for residual gold, 

3, Wash the gold sponge with distilled 
water and dry at 100° C, The dry gold 
sponge should assay between 997 and 999 
fine. 



RECOMMENDED PROCEDURE FOR STEEL WOOL CATHODES 



HCl pretreatment of the cathodes is re- 
quired to dissolve the steel wool. One 
kilogram of steel wool cathode is slowly 
added to 10 L of 5M HCl, which is heated 
to 90° C while stirring. The slurry is 
digested at temperature for 1 h, filtered 
hot, and washed with water. 

The HCl-pretreated cathode residue is 
leached with dilute aqua regia to recover 
the gold and silver. The HCl leach resi- 
due is divided into two equal charges. 
One charge is slurried with dilute aqua 
regia. For every 100 g of charge, 2 L of 
dilute aqua regia containing 150 mL of 
36.5-pct HCl and 100 mL of 70-pct HNO3 
are used. The pulp is heated to 90° C, 
digested, and agitated for 1 h. The hot 
slurry is filtered on glass fiber filter 



paper, and the acid filtrate is removed. 
Displacement washing of the residue with 
distilled water is continued until no 
yellow color is visible in the filtrate. 
The washes are combined with the acid 
filtrate. The residue from the first 
aqua regia leach is an AgCl product which 
should contain 70 to 74 pet silver. The 
second charge of HCl-leached residue is 
slurried with the combined filtrate and 
treated as above. The second aqua regia- 
leached residue is saved for further di- 
lute aqua regia leaching. 

The procedure for precipitating me- 
tallic gold from the resultant solution 
with (C00H)2 is the same as for zinc 
precipitates. 



HANDLING AND DISPOSAL OF ACID SOLUTIONS AND ACID WASTES 



The acid solutions used are corrosive 
and should be handled with extreme care. 
All leaching must be done in vessels that 
will not be corroded. The solutions 
should not contact the skin or eyes. 
Protective clothing should be worn to 
prevent accidental contact. Should the 
acid solution contact a person's skin or 
eyes, flush with cold water for a minimum 
of 15 min and consult a physician. 



The fumes from the acid solutions and 
chemical reactions with the solutions can 
be very hazardous. All operations should 
be conducted in a fume hood or in a well- 
ventilated area. If a fume hood is not 
available, ensure good outside ventila- 
tion and wear an appropriate chemical 
respirator and full protective clothing 
at all times. 



11 



The acid waste solutions should be neu- 
tralized with alkaline solutions before 
disposal. Violent reactions may occur 
when mixing acid solutions with alkaline 
solutions. Extreme care should be taken. 
The solutions should be slowly mixed to 



prevent an uncontrollable reaction. The 
pH of the neutralized acid waste should 
be between 6 and 7. The neutralized 
solutions should be disposed of accord- 
ing to local, State, and/or Federal 
regulations. 



SUMMARY AND CONCLUSIONS 



The experimental results showed that 
zinc precipitates and steel wool cathodes 
can be chemically refined to produce a 
pure metallic gold and an AgCl precipi- 
tate. Chemical refining of zinc precipi- 
tates and steel wool cathodes recovered 
99.9 pet of the contained gold. HNO3 
preleaching separated silver and base 
metals from the gold in zinc precipi- 
tates. Silver in the HNO3 leaching solu- 
tion was recovered as AgCl by precipita- 
tion with NaCl. HCl leaching separated 
base metal from the gold and silver con- 
tained in steel wool cathodes. Silver 



was recovered as an AgCl precipitate 
which contained some gold. 

Aqua regia leaching dissolved gold in 
the residues from HNO3 and HCl preleach- 
ing. (C00H)2 and SO2 precipitated metal- 
lic gold from the aqua regia leaching 
solutions. The gold produced by (C00H)2 
reduction was 998 to 999 fine. Gold pro- 
duced by SO2 reduction was 970 to 988 
fine, but was upgraded to 998 fine by di- 
gestion in HNO3 . The gold can be puri- 
fied to 999.9 fine by fire refining. 



REFERENCES 



1. Adams on, R. J. Gold Metallurgy in 
South Africa. Chamber of Mines of South 
Africa, 1972, pp. 203-255. 

2. Hall, K. B, Homes take Carbon- 
In-Pulp Process. Pres. at Am, Min, Cong. 
Mtg., Las Vegas, NV, Oct. 7-10, 1974, 16 
pp.; available upon request from Home- 
stake Mining Co., Lead, SD. 

3. Heinen, H. J., G. E. McClelland, 
and R. E. Lindstrom. Enchancing Perco- 
lation Rates in Heap Leaching of Gold- 
Silver Ores. BuMines RI 8388, 1979, 
20 pp. 

4. Heinen, H. J. , D. G. Peterson, and 
R. E. Lindstrom. Processing Gold Ores 
Using Heap Leach-Carbon Adsorption Meth- 
ods. BuMines IC 8770, 1978, 21 pp. 

5. Hinds, H. L. , and F. Mosely. Re- 
fining Gold at Homes take. Undated, 8 pp; 
available upon request from Homestake 
Mining Co., Lead, SD. 

6. Hoke, C. M. Refining Precious 
Metal Wastes. Metallurgical Publ. 
Co., New York, 1940; 355 pp.; re- 
prints available from Univ. Microfilms 



International, 300 North Zeeb Road, Ann 
Arbor, MI 48106. 

7. Lowen, R. The Worshipful Company 
of Goldsmiths (London), Tech. Rept. 44/1, 
1980, 22 pp. 

8. McClelland, G. E., and J. A. 
Eisele. Improvements in Heap Leaching To 
Recover Silver and Gold From Low-Grade 
Resources. BuMines RI 8612, 1982, 26 pp. 

9. Merrill-Crowe. Merrill-Crowe Pre- 
cipitation Process — Details of Operation. 
Undated, 9 pp.; available upon request 
f rom M. D. Wroblewski, BuMines, Reno, NV. 

10. Pickett, D. E. Milling Practice in 
Canada. Can. Inst. Min. and Metall. , 
spec. V. 16, 1978, pp. 45-78. 

11. Potter, G. M. Design Factors for 
Heap Leaching Operations. Min. Eng, , 
Mar. 1981, pp 277-281. 

12. Zadra, J. B. , A. L. Engel, and 
H. J. Heinen. Process for Recovering 
Gold and Silver From Activated Carbon 
by Leaching and Electrolysis. BuMines 
RI 4843, 1952, 32 pp. 



*U.S.CPO: 1985-505-019/5092 



INT.-BU.OF MINES, PGH., PA. 27847 



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